Time and place. Let's get some context.
Two months ago, this man discovered that with a strange effort of will, a burst of peculiarly chitinous cold rage, he could turn anything he wanted into a dense mass of spiders. Fortunately, his rage was aimed only at a piece of furniture on the day he figured it out; and he disguised the disappearance of the dubious flat-pack chair as well as he could.
Three days ago, let's say, his girlfriend worked out where all the spiders in the house came from. An inadvisable word dropped into a conversation, an unconvincing lie or two, the truth coming out. A demanded demonstration. Another piece of furniture dissolves into lively black liquidity. A running up the stairs; a slammed door. "Darling? Darling?". Then the shouting starts.
Sixty seconds ago, the fury and terror was still raging. Neither had slept since the whole mess had started. This time the argument got so hot that she tried to hit him with an old cast-iron pan. She missed, falling onto the floor. He saw red. Then he saw black.
That's enough to understand the now. Because this, in a damp and melancholy cellar somewhere under suburban London, is a very unusual murder-suicide.
You see, when something turns into a mass of spiders, the spiders in the middle can't get out for quite some time, until the spiders above them have moved, and those spiders can't move until the ones above them have moved, and so forth, all the way up to the topmost. This means that objects keep their shape for a surprising amount of time.
There are two figures lying on the floor. Look at them. These fingers are black and shivering with the confusion of a thousand tiny creatures. These eyelashes, still nearly perfect, are the legs of creatures who cannot yet escape. The detail of the nose is already disappearing; the spiders whose legs were the hair of their arms and legs and chest have already righted themselves and made for safer places. There is a quiet rustling sound as the creatures move.
In about ten seconds time, their faces and their limbs will be well on the way to dissolution, streams of blackness running down from the places that had been their cheekbones. Fingers and hands liquefying into black points. Dark rivers running from the unbodies to the shadows and corners of the room. After two minutes, the floor will be empty and clean.
In about two days, the neighbours will start looking for the nest.